Magnetic nanocrystals are widely used in different types of biological applications such as DNA/RNA, protein, and cell separation and purification, and biomedical applications such as magnetic resonance imaging (MRI), hypothermia treatment of cancer, and drug delivery, etc. However, the biocompatibility, surface functionality, chemical stability, and colloidal stability of the magnetic nanocrystals under physiological conditions remain the bottlenecks for the abovementioned applications.
At present, the chemical synthetic methods for magnetic nanocrystals and nanoparticles mainly rely on hydrolysis or pyrolysis of metal compounds, such as (co)precipitation method, thermal-decomposition method, microemulsion method, sonochemical method, and so on. The nanoparticles prepared by (co)precipitation method have a wide particle size distribution and less-defined composition; the nanoparticles prepared by microemulsion method show low crystallinity degree and weak magnetic responsivity; the sonochemical method shows poor ability in controlling the size and the morphology of the resultant nanoparticles. However, thermal-decomposition method developed recently has successfully overcome the above-mentioned problems. As higher reaction temperature are usually adopted in the thermal decomposition method, the nucleation process, growth process and the crystallinity degree of the resultant nanocrystals can better be controlled. On the other hand, the use of non-polar or weak polar organic compound as reaction medium can prevent water from being involved in the reaction, which is greatly helpful for defining the composition of the resultant nanocrystals. From the following literatures, i.e., Rockenberger J, Scher E C, Alivisatos A P. A New Nonhydrolytic Single-Precursor Approach to Surfactant-Capped Nanocrystals of Transition Metal Oxides. J. Am. Chem. Soc. 1999, 121(49): 11595-11596; Jana N R, Chen Y, Peng X. Size- and Shape-Controlled Magnetic (Cr, Mn, Fe, Co, Ni) Oxide Nanocrystals via a Simple and General Approach. Chem. Mater. 2004, 16(20): 3931-3935; Park J, An K J, Hwang Y S, Park J G, Noh H J, et al. Ultra-large-scale syntheses of monodisperse nanocrystals. Nature Materials 2004, 3(12): 891-895; Hyeon T, Lee S S, Park J, Chung Y, Bin Na H. Synthesis of highly crystalline and monodisperse maghemite nanocrystallites without a size-selection process. J. Am. Chem. Soc. 2001, 123(51): 12798-12801; Sun S H, Zeng H. Size-controlled synthesis of magnetite nanoparticles. J. Am. Chem. Soc. 2002, 124(28): 8204-8205, it can be found out that the solvent for preparing high-quality magnetic iron oxide particles is typically chosen from non-polar and weak polar organic compounds with a high boiling point. In addition, small molecules such as fatty acids, fatty amines or fatty alcohols are typically presented in the reaction system. The above-mentioned investigations have formed a solid basis for preparing high quality magnetic nanocrystals. However, the direct products of the above mentioned preparations involving thermal decomposition method are typically characterized by a satisfying organic dissolvability due to the hydrophobic surface modification by small alkyl molecules. Therefore, it is impossible to directly use them at single particle level for in vivo applications. Although the hydrophobic magnetic nanocrystals can be transferred into an aqueous solution via a ligand-exchange process, the post-preparative procedures are very complicated and laborious.
Recently, Mingyuan Gao's group from the Institute of Chemistry, Chinese Academy of Sciences further developed the thermal-decomposition method by adopting high boiling point strong-polar solvent as reaction medium as well as a coordinating solvent and established a one-pot reaction technique for producing water soluble magnetic nanocrystals (Chinese patent: 03136275.3 and 200610114459.X). In their technique, the weak polar and non-polar solvents were replaced by a strong polar solvent such as 2-pyrrolidone. For example, by pyrolyzing ferric triacetylacetonate (Li Z, Chen H, Bao H B, Gao M Y. One-pot reaction to synthesize water-soluble magnetite nanocrystals. Chem. Mater., 2004, 16(8): 1391-1393) or FeCl3.H2O (Li Z, Sun Q, Gao M Y. Preparation of water-soluble magnetite nanocrystals from hydrated ferric salts in 2-pyrrolidone: Mechanism leading to Fe3O4. Angew. Chem. Int. Ed, 2005, 44(1): 123-126) in 2-pyrrolidone, they have successfully obtained water soluble magnetite nanocrystals. On the basis of these achievements, they further developed a one-pot reaction technique for producing biocompatible magnetic nanocrystals by introducing carboxyl-terminated polyethylene glycol into the reaction system (Chinese patent: 03136273.7). By this technique water soluble, biocompatible (Li Z, Wei L, Gao M Y, Lei H. One-pot reaction to synthesize biocompatible magnetite nanoparticles. Adv. Mater. 2005, 17(8): 1001-1005) and biocompatible magnetic nanocrystals bearing surface reactive carboxyl group (Hu F Q, Wei L, Zhou Z, Ran Y L, Li Z, Gao M Y. Preparation of biocompatible magnetite nanocrystals for in vivo magnetic resonance detection of cancer. Adv. Mater., 2006, 18(19): 2553-2556) were successfully obtained by one-pot reaction. The biocompatible magnetite nanocrystals prepared by the technique described in the Chinese patent 03136273.7 present very good colloidal stability. Furthermore, the resultant biocompatible particles in powder form also present satisfying dissolvability in pure water. But their dissolvability and colloidal stability in physiological buffers remains to be improved. Therefore, on the basis of the Chinese patent 03136273.7, herein we further developed the one-pot reaction technique for producing biocompatible magnetic nanocrystals with better dissolvability and colloidal stability in physiological buffer. In comparison with the technique described in patent 03136273.7, the current invention however adopts nonpolar and weak polar solvent to replace the strong polar coordinating solvent. Moreover, small alkyl molecules which can coordinate with the metal ions on the surface of the magnetic nanocrystals are also present in the reaction system, apart from biocompatible macromolecules which can be chemically modified on the surface of the resultant nanocrystals. Consequently, the resultant biocompatible nanocrystals exhibit a much higher solubility and greatly improved colloidal stability in physiological buffers in comparison with the magnetite nanocrystals obtained by patent 03136273.7.
In addition, user-friendly techniques for chemically conjugating the biocompatible magnetic nanocrystals to biomolecules remain to be developed for further expanding the biomedical applications of the magnetic nanocrystals.
At present, there are mainly two kinds of methods for conjugating inorganic nanocrystals to biomolecules: 1) the first group of methods relies on the weak interactions such as electrostatic adsorption, hydrophobic interaction, the coordination interactions between metal ions on the magnetic nanocrystals and histidine residues on the biomolecules, and so on; 2) the second group of methods relies on covalent bonding between the reactive moieties from the surface capping agents on the magnetic nanocrystals and the reactive residues on the biomolecules. Compared with weak interactions, covalent bonding strategy leads to conjugate with higher stability and clearly defined conjugation structure.
At this moment, the EDC/Sulfo-NHS (1-Ethyl-3-(3-dimethylaminopropyl)carbodiimide/N-hydroxysulfosuccinimide sodium salt) mediated amidation reaction has widely used as a standard covalent coupling reaction. Different types of EDC and NHS (N-hydroxysuccinimide) derivatives have also been synthesized and used for this purpose (Bioconjugate Techniques, Academic Press, New York, (1996) p 173-176; Adv. Mater., (2006) 18:2553-2556). One of the prominent advantages of this coupling method is that the conjugation reaction can be performed under mild conditions. However, the resultant N-hydroxysuccinimide ester moiety, as an intermediate product for further reacting with amino-bearing compounds, is very readily to hydrolyze. Therefore, the activation of the carboxyl group and the following conjugation reaction with amino-bearing biomolecules, mediated by EDC/Sulfo-NHS, have to be performed simultaneously or successively within a short time window. This is for many practical applications very inconvenient.